The electro-optical inspection apparatus is used for obtaining an image of a sample surface of a semiconductor wafer or the like by irradiating the sample surface with a primary electron beam, and detecting secondary electrons or mirror electrons emitted from the sample surface, to perform inspection of defects on the sample surface and/or pattern evaluation of the sample surface on the basis of the obtained image.
In addition, it is known that while a sample such as a semiconductor wafer is conveyed in the air or in vacuum for inspection or processing, if adhesion of dusts such as particles having a diameter of 100 nm or smaller to the sample surface can be prevented to reduce adhesion of dusts or particles to be killer defects on a pattern to the sample surface, production yield can be greatly improved. In a semiconductor or LSI manufacturing process, if the killer defects are on the sample surface, a width of wiring or insulation resistance becomes insufficient so that performance is significantly deteriorated. Therefore, it is necessary to take measures against the killer defects, such as correction or detection of the defects.
In particular, the electro-optical inspection apparatus can provide a measurement or inspection result with high accuracy by reducing adhesion of dusts or particles generated by the inspection apparatus to a sample surface. Thus, a process of dusts or particle adhesion to the sample surface can be identified and improved so that defects in an exposure process can be reduced, for example.
Conventionally, in relation to conveyance of samples, measures have been taken for reducing generation of dusts or particles as much as possible. For instance, as to an air conveyance system, a mini-environment having a high-performance filter and down flow is provided, and the air conveyance system is housed in the mini-environment. Further, a static electricity remover removes static electricity of the sample so as to prevent particles from adhering to the sample surface. In addition, in a vacuum conveyance system for samples, control of fluctuation of pressure is performed by a load lock. However, it is usually difficult to reduce dusts or particles having very small sizes, in particular, of 100 nm or smaller.
In view of this problem, the applicant has proposed a method of removing dusts or particles from a sample surface (refer to Japanese Patent Application Publication No. 2009-4161 (Patent Document 1)). In this method, the dusts or particles on the sample surface are detected, the sample is moved in a horizontal direction, a collection electrode disposed close and opposite to the sample surface is charged to have the polarity opposite to the charge polarity of the dusts or particles, and hence approaching dusts or particles are electro-statically attracted by the xollection electrode. Thus, the dusts or particles on the sample surface are detected, and the detected dusts or particles are removed from the sample surface.
In addition, there is also proposed a particle monitor (refer to Japanese Patent Application Publication No. 2010-56270 (Patent Document 2)), in which particles floating in a plasma processing apparatus are collected actively by a dusts or particle collecting electrode and are accumulated in a specific place using a quadrupole linear trap or the like so that the particles are detected electrically or by using laser scattered light.
As a design rule of LSI has become very fine, a size of dusts or particles to be prevented from adhering to a sample surface has become very small. Adhesion of dusts or particles generated by an operation of the inspection apparatus to the sample surface has become a critical problem, and it is necessary to take a countermeasure.
However, in the conventional dusts or particle adhesion prevention mechanism, it is difficult to prevent dusts or particles such as fine particles of 100 nm or smaller, for example, from adhering to the sample surface. In particular, if a sample is placed on a stage or the like in a vacuum chamber for inspecting the sample surface by the electro-optical inspection apparatus, there has been no consideration about suppressing generation of dusts or particles from the apparatus itself.
The prior art described in Patent Document 1 detects that dusts or particles have adhered to the sample surface, and afterward removes the adhered dusts or particles from the sample surface. Therefore, conventional example described in Patent Document 1 cannot prevent dusts or particles from adhering to the sample surface. In addition, in the prior art described in Patent Document 2, the dusts collecting electrode is provided to the tip of the particle monitor, for collecting particles floating in the plasma processing apparatus. Therefore, adhesion of particles to the sample surface cannot be prevented.
In addition, there is a case that, when a vacuum chamber is evacuated, air flow causes static electricity, which electrifies dusts or particles remaining in the vacuum chamber, and the electrified dusts or particles such as particles are electro-statically attracted by the sample surface to be inspected and adhere thereto so that the sample surface is contaminated. A conventional vacuum chamber used for inspecting a sample surface has no countermeasure against the electrostatic attraction of dusts or particles remaining in the vacuum chamber to the sample surface, and cleaning has been the only way to cope with such remaining particles. Therefore, it is strongly required to prevent such remaining particles in the vacuum chamber that could not be removed by cleaning from adhering to the sample surface.
The present invention has been made in view of the above-mentioned problem, and a first object thereof is to provide a method of preventing as many dusts or particles as possible from adhering to a sample surface, and an electro-optical inspection apparatus for inspecting a sample surface using an electron beam while preventing as many dusts or particles as possible from adhering to the sample surface.
Japanese Patent Application Publication No. 2005-235777 (Patent Document 3) describes a method of observing a sample surface using an electro-optical inspection apparatus. This conventional method utilizes gradation differences of an image generated in a part where an open defect or a short defect exists. In the part where a defect such as an open defect or a short defect exists, gradation differences appear in the image, which never appear in a normal part. Therefore, in the conventional method, a wafer surface image obtained from a semiconductor wafer surface is compared with an original surface image (of a wafer without any defect). If a gradation difference, which must not appear when using a normal wafer, appears in a part, it is determined that the part has an open defect or a short defect.
However, in an observation method disclosed in Patent Document 3, there is a problem that the gradation difference in an observation part is originally small depending on a structure or material of the sample to be observed, and that it is difficult to detect a short defect or an open defect in some cases.
In addition, particularly in the case of the open defect, the image of the part where the open defect exists becomes darker (blacker) than the image of the normal part in one case, and becomes lighter (whiter) than that of the normal part in another case. Therefore, there has been a problem that it is very difficult to detect a defect and/or to classify a type of the defect.
In view of this problem, the inventors of the present invention have studied to provide a sample surface observation method that can obtain a sample surface image having large gradation differences between a defect part and a normal part in defect detection of a wiring structure and a clear difference between gray levels of white and black, so that a defect can be easily detected. As a result, the inventors of the present invention have found that the problem of the sample surface observation method disclosed in Patent Document 3 is due to simultaneous detection of a short defect and an open defect in the same condition. Hence, they have proposed a new sample surface observation method (see Japanese Patent Application Publication No. 2009-87893 (Patent Document 4)).
Patent Document 4 discloses a sample surface observation method in which an electron beam irradiates a sample surface on which a wiring pattern containing insulation material and conductive material is formed, and hence electrons having structural information of the sample surface are detected so that an image of the sample surface is obtained for observing the sample surface. The sample surface is irradiated by the electron beam in the state where the insulation material and the conductive material have the same luminance in the sample surface image so that a part other than the insulation material and the conductive material can be easily and reliably detected. In addition, a point having a luminance which is different from the luminance of the insulation material and the conductive material in the sample surface image is detected as an open defect on the sample surface, so that the open defect can be easily and reliably detected.
In addition, Patent Document 4 also discloses a sample surface observation method that can effectively detect a short defect, in which an electron beam irradiates a sample surface in a state that a luminance difference between the insulation material and the conductive material becomes maximum in the sample surface image, and hence the sample surface image that facilitates distinction between the short defect and its surrounding part is obtained, so that the short defect can be easily and reliably detected. In addition, a state that the luminance difference becomes the maximum is determined in a mirror electron region in which electrons having the structural information of the sample surface become mirror electrons.
However, the inventors of the present invention have further studied a method and apparatus that enables observation of a sample surface with high contrasts in a case where an insulation region and a conductive region are formed on the sample surface, and facilitates not only detection of a short defect or an open defect but also classification of a type of the defect. As a result, the inventors of the present invention have reached a conclusion that the method disclosed in Patent Document 4 still has room for improvement.
Therefore, a second object of the present invention is to provide a sample observation method and apparatus that can achieve the above-mentioned first object, and further enables high contrast observation of a sample surface on which an insulation region and a conductive region are formed, and facilitates detection of a short defect or an open defect as well as classification of a type of the defect, and to provide a sample inspection method and apparatus using the sample observation method and apparatus.